The use of bone screws for medical procedures, such as the repair of bone fractures, is known. For example, U.S. Pat. No. 4,463,753 describes a bone screw having distal threads and proximal threads. That bone screw is specifically intended for repair of a fracture by compressing the fractured segments of a bone while maintaining proper bone alignment. An end of the screw is provided with a hexagonal bore for engaging an Allen wrench, which can then be used for inserting the bone screw. However, for purposes of removing the bone screw, an Allen wrench cannot be used for exerting a pulling force for forcibly removing the bone screw.
One technique for addressing this problem is described by U.S. Pat. No. 5,375,956, which discloses a bone screw having a smooth shaft with threads near the head of the screw, as well as reverse threads extending about the head of the screw for engaging an internally-threaded extraction device. The screw can be removed by driving the extraction device onto the reverse threads to an engagement limit, at which point the extraction device can be further driven for partial extraction of the screw from the bone. Once the threads have been backed out of the bone, the screw is pulled until the remaining smooth shaft is extracted. The engagement of the internal threads of the extraction device with the external threads of the screw head allows the extraction device to be used for exerting a pulling force for extracting the smooth shaft of the screw from the bone.
However, the internally-threaded screw extractor still presents several drawbacks. For example, the use of the internally-threaded screw extractor requires the use of a screw having a threaded screw head, thus adding to the complexity and expense of manufacturing the bone screw. In addition, tissue growth often occurs in the threads of the screw head while it is in place, making it difficult to thread the extractor onto the screw for removal.